The invention relates to polishing a semiconductor substrate by a polishing pad having a transparent portion for transmission of an optical beam that is reflected from a surface of the semiconductor substrate, while the substrate is being polished.
Polishing of a semiconductor substrate is further known as, polishing by chemical mechanical polishing or chemical mechanical planarization, CMP. Such polishing by CMP removes a layer of metal from an underlying barrier film of metal, and further removes the barrier film from an underlying dielectric layer on the semiconductor substrate, which leaves a smooth, planar polished surface on the dielectric layer serving as a substrate on which successive layers of material are fabricated, and further which leaves electrical circuit interconnects of precise dimensions, which interconnects are imbedded in trenches that are flush with the polished surface.
U.S. Pat. No. 6,074,287 discloses a known polishing pad for polishing a semiconductor substrate. Such polishing is performed by a polishing surface on the polishing pad, together with a fluid polishing composition at an interface of the polishing pad and the semiconductor substrate. The known polishing pad has at least a portion that is transparent to an optical beam, to transmit the optical beam for incidence of the optical beam on the surface of the semiconductor substrate. The optical beam transmits an optical signal that becomes incident on the surface of the semiconductor substrate, and that reflects from the surface. The optical signal becomes encoded with information pertaining to a change in the thickness of a layer as it is being removed by polishing. The reflected optical beam emanates to a detector that monitors the optical signal for such information. Further, the detector monitors the optical signal for end point detection, EPD, the occurrence of an end point, at which further polishing is stopped because the detector detects that substantial removal of the layer has been completed.
A problem arises because the polishing surface of the polishing pad varies from being flat and smooth. The polishing surface is manufactured with a topography of peaks and valleys, which includes, for example, a roughened surface texture, and a grooved surface. Such a topography enhances polishing abrasion, and distributes the fluid polishing composition evenly at the interface of the polishing pad and the semiconductor substrate. The peaks and valleys on the surface of the polishing pad change the orientation of the surface, such that the optical beam is incident on such a surface at different angles, depending on the orientation of such a surface at the point of incidence. Further, the surface of the polishing pad can become worn by polishing the semiconductor substrate, which changes the peaks and valleys, and which changes the orientation of the surface. The varying topography of the polishing surface scatters the incident optical beam. Scattering is undesirable for causing attenuation of the optical signal, and misdirection of the optical beam.
Another problem arises because the polishing pad and the fluid polishing composition meet along an interface thereof, which tends to scatter the optical beam due to the polishing pad and the fluid polishing composition having mismatched angles of refraction at the interface. Thus, a need exists for minimizing scattering of an optical beam at an interface with a transparent portion of a polishing pad.
The invention minimizes scattering of an optical beam at an interface with a surface of a transparent portion of a polishing pad. The invention includes a polishing pad for polishing a semiconductor substrate, the pad having at least a portion that is transparent to an optical beam, and the index of refraction of the transparent portion matched nearly to, differing by no more than a few percent of, that of a fluid polishing composition that is used during polishing of the semiconductor substrate, which minimizes scattering of the optical beam.
Further, the invention includes a method for adjusting the index of refraction of a fluid polishing composition for polishing a semiconductor substrate, comprising: adding a nonreactive salt in solution to adjust the index of refraction of the polishing composition matched nearly to, by no more than a few percent of, the index of refraction of a transparent portion of a polishing pad.
The invention further includes a fluid polishing composition for polishing a semiconductor substrate, the index of refraction of the polishing composition being adjusted to be matched nearly to, by differing by no more than a few percent of, that of a transparent portion of a polishing pad that is used during polishing of the semiconductor substrate, which minimizes scattering of an optical beam at an interface of the polishing composition and the transparent portion.